DC-DC converters have assumed, and continue to assume, increasing importance in a plurality of electronic systems and devices, such as, for example, portable devices like cell phones and laptop computers, which are supplied via batteries. Said electronic devices usually include a plurality of electrical circuits and subcircuits, each of which requires, for its own operation, a level of voltage different from the one supplied by the battery. Furthermore, the voltage of the battery decreases during use thereof, as the power is drawn off. DC-DC converters are used in such applications in so far as they enable a method of generation of one or more controlled voltage levels starting from a variable supply voltage, such as, for example, the voltage supplied by a battery. An advantage of using a DC-DC converters may be a considerable saving of space in so far as it makes up for the need to use as many different supply sources as are different voltage levels required by the various electronic circuits for their operation. In particular, electronic switch-mode DC-DC converters carry out a DC-DC conversion by applying a DC voltage across an inductor for a predetermined period of time (usually in a frequency range of approximately from 100 kHz to 5 MHz) so as to generate a flow of electric current and store magnetic energy in the inductor. When, then, the voltage across the inductor is removed, the energy stored is transferred as output voltage and current of the DC-DC converter in a controlled way. By acting on the duty cycle, i.e., the time ratio of switching-on/switching-off, the output voltage remains regulated even though the load current may vary. This method of conversion is very power efficient (the efficiency is typically between approximately 80% and 95%), unlike other solutions, such as linear methods of conversion, which dissipate more power.
The external passive components, such as inductors and capacitors used in integrated DC-DC converters, have a value of impedance that depends upon the switching frequency and upon the power that the converter is designed to supply. In the last few years, DC-DC-converter designers have been increasing said switching frequency so as to be able to reduce the dimensions of the inductors to values such that they can be integrated in the converter package (in order to obtain the so-called “Systems in Package”, or SiPs), or even integrated in the die or chip itself (the so-called “Systems-on-Chip”, SoCs). Reference may be made, for example, to Saibal Roy “Challenges in magnetics for PwrSoC—Development in highfrequency magnetics, materials and integration”, PWR'SoC 2010 Cork, Ireland, which is incorporated by reference.
As is known, inductors are formed by a low-resistance metal winding, which surrounds a core, which may be made of magnetic material. To provide a microinductor or an integrated inductor, it is typically necessary to have available low-resistivity metal paths and a magnetic material in the form of thin film. In order to maximize the quality factor Q of the inductor, the magnetic material has low coercivity, high saturation, and good response at high frequencies (>10 MHz). Another requisite of the magnetic material for it to be integrated in current micromachining processes is for it to be compatible with silicon processes. Various materials have been studied and many others are still under study (see, for example, Nian X. Sun “RF Magnetic Films and Their Applications in Integrated Magnetic Devices”, PWR'SoC 2010 Cork, Ireland, which is incorporated by reference.
There currently exist a plurality of different approaches for providing an integrated inductor, the best known of which envisages forming a planar loop surrounded by two thin layers of magnetic material defined lithographically. Said method is, for example, known from Donald S. Gardner “Integrated On-chip Inductors With Magnetic Films”, IEEE Transactions On Magnetics, Vol. 43, No. 6, June 2007, which is incorporated by reference.
The methods of manufacture of integrated inductors of a known type envisage complete integration of the magnetic material on the die of the DC-DC converter, and thus require that the processes of machining of the magnetic material do not come to affect the previous processes of machining of silicon, in effect reducing the choice of the magnetic materials that can be used to a limited group.